1. Field of the Invention
The invention relates generally to image processing and, more specifically, to a system and method for adaptively sharpening digital images.
2. Description of the Related Art
Digital camera technology has developed rapidly over the past decade. A broad variety of digital cameras are now available to meet the diverse needs of a wide spectrum of consumers. Digital camera technology provides a user with the ability to capture digital images and/or digital video, and also provides the ability to transfer the recorded digital images and/or digital video to a computer-readable medium, such as a Digital Video Disc (DVD) or a computer memory, or to a physical medium, such as photographic paper.
Additionally, once image data and/or video data is captured by the digital camera, the data may be transferred to a computer system via a cord or connector that couples the digital camera to the computer system. Common types of connectors provided with digital cameras are universal serial bus (USB) connectors, firewire connectors, High-Definition Multimedia Interface (HDMI) connectors, proprietary connectors, or other types of connectors that may be used to transfer data. Some digital cameras may include a cord that connects the digital cameras to the computer system; whereas, other digital cameras may include a connector that protrudes from the digital camera and can be plugged directly into the computer system.
As is known, video data is comprised of a series of digital images. Each digital image is represented by a two-dimensional (2D) grid of pixels that may be displayed on a screen, such as a television set or a computer monitor, or printed on high-quality paper to provide a photographic still image. The color and brightness of the pixels can be represented using different formats. For example, each pixel may be defined by three values that define the brightness of red, green, and blue (RGB) color components associated with the pixel. The brightness value of each color typically falls between 0 and 1, where 0 represents a lack of the color, and 1 represents the maximum brightness of the color. Digital images that are represented as RGB values are referred to herein as “RGB images.” Each pixel of the digital image may also be defined by a luminance component (Y) that defines the brightness of the pixel, and two chrominance components (U and V) that define the color of the pixel. Digital images composed of pixels that are represented with YUV components are referred to herein as a “YUV images.”
RGB images and/or YUV images may be filtered to produce higher-quality digital images. For example, a digital image of either format may be sharpened using a sharpening filter to produce data that, when displayed on a screen, appears clearer. A typical sharpening filter amplifies the differences in brightness from pixel-to-pixel in a digital image. For example, a sharpening filter may modify the luminance component of a YUV image to provide a sharper YUV image. One problem with this approach is that typical sharpening filters often introduce a variety of distortions into the digital image. These distortions are known as “sharpening artifacts” that may be introduced into the digital image under several different circumstances.
For example, sharpening artifacts known as “bright overshoot” and “dark undershoot,” known collectively as “overshoot,” may be introduced into the digital image by the sharpening filter within regions of the digital image that rapidly transition from a bright area to a dark area. Such a region may be indicative of an edge between different objects represented by the digital image. This effect is illustrated in FIG. 9. As shown, FIG. 9 includes graph 900 of horizontal pixel position versus luminance based on a YUV image, according to the prior art. Original edge 910, represented by a solid line, indicates that the luminance of the YUV image decreases from a first value to a second value across the horizontal line of pixels. The region of the YUV image where this decrease occurs is shown on graph 900 as extended high gradient region 930.
After the YUV image is sharpened, the luminance value decreases more rapidly from the first value to the second value across extended high gradient region 930. However, the luminance value is also increased at the left-most extremity of the extended high gradient region 930, resulting in bright overshoot, and decreased at the right-most extremity of the high gradient region 930, resulting in dark undershoot. Bright overshoot causes excessively bright pixels to appear within a bright region that is near a boundary with a dark region, while dark undershoot causes excessively dark pixels to appear within a dark region that is near a boundary with a bright region. These excessively bright and dark pixels may disrupt the overall quality of the image and may be visually displeasing.
Another type of artifact is known as “color fringe.” When the RGB values of the RGB image are in focus, as shown in FIG. 10A, the RGB brightness values each change at the same rate across a boundary between a bright region and a dark region. However, when the RGB values are not in focus, as shown in FIG. 10B, these brightness values may change with different rates across such a boundary, resulting in a fringe near the boundary. Within this fringe, one color appears brighter than the other colors. This may be caused by, among other things, flaws in the lens of the DVC. The brightness of the fringe may be amplified when the image is sharpened due to the effects of overshoot described above, further exacerbating the color fringe and reducing the overall quality of the image.
A third type of artifact, referred to herein as a “dark artifact,” occurs when a dark region of a digital image is sharpened. Dark regions of a digital image may include random noise that, when sharpened, appears more pronounced. This amplified noise disrupts the continuity of the dark region and decreases the overall clarity of the digital image.
Accordingly, there remains a need in the art for a technique to sharpen digital images without introducing sharpening artifacts.